The density of gaseous hydrocarbons, one of substances causing photochemical smog, that are released into the atmosphere is strictly regulated by law not only in such advanced countries as the United States, European countries and Japan but also in Taiwan, Mexico, China, and so forth.
Although specific levels of regulatory standards vary depending on the situation of each country, the advanced countries, except for Japan, adopt the criterion of 1 vol % or less (38 mg/l or less) set by the United States Environmental Protection Agency (EPA). Among them, the United States and European countries strictly comply with this criterion. A recent trend has been to make the regulatory standards even more stringent.
Waste gases which are released when landing volatile hydrocarbons for storage or when loading or unloading them during transportation is a matter of particular concern as a source of gaseous hydrocarbons. This concern involves tank trucks, railroad freight cars, storage tanks and tankers engaged in domestic trade, for example.
Widely known conventional techniques for treating and recovering such waste gases containing gaseous hydrocarbons include:
(1) Absorption method (normal pressure absorption and reduced pressure recovery method) based on the use of a nonvolatile hydrocarbon solvent;
(2) Membrane gas separation method based on the use of a gas separating membrane;
(3) Low temperature processing method in which waste gas is cooled to an extremely low temperature; and
(4) Adsorption method based on the use of activated carbon or zeolite.
Among the aforementioned methods (1)-(4), an example of the absorption method (1) is disclosed in Japanese Examined Patent Publication No. 54-8632, in which waste gas is passed through, or washed out with, a nonvolatile hydrocarbon solvent, causing volatile hydrocarbons contained in the waste gas to be dissolved, and remaining air which is insoluble in the solvent is released into the atmosphere. The solvent containing the volatile hydrocarbons is then flashed in a vacuum vessel to separate volatile hydrocarbon vapor and the nonvolatile hydrocarbon solvent from each other. In this method, the separated volatile hydrocarbons are recovered by washing them with a hydrocarbon liquid of the same properties while the nonvolatile hydrocarbon solvent also recovered in the separation process is recycled for further use. This method is most widely used in Japan.
The method mostly commonly used in other countries is the aforementioned adsorption method (4) which employs activated carbon. Specific examples of the activated carbon adsorption method are disclosed in Japanese Unexamined Patent Publications Nos. 57-14687 and 57-42319 as well as in Japanese Examined Patent Publications Nos. 59-50715, 59-50716 and 2-46630. Compared to the earlier proposed absorption method (1) mentioned above, the activated carbon adsorption method is almost same as the absorption method in system configuration and is regarded as a technique which differs from the absorption method merely in that a process of absorbing gaseous hydrocarbons by the nonvolatile hydrocarbon solvent is replaced by an adsorption process using activated carbon. Accordingly, the aforementioned absorption method (1) can be regarded as a method in which a liquid adsorbent is used in a fluidized bed.
The aforementioned absorption method (1) has one drawback, however, in that the vacuum vessel must be depressurized to the degree of vacuum of about 3 mmHg to meet the United States EPA regulations, because the concentration of gaseous hydrocarbons remaining in air which is released into the atmosphere is determined uniquely by the degree of vacuum of the vacuum vessel.
However, a vacuum pump capable of processing waste gases at a rate of a few hundred cubic meters per hour at such a high vacuum is not presently available. It is therefore impossible to comply with the EPA regulations by the aforementioned absorption method (1).
On the other hand, the aforementioned adsorption method (4) can easily comply with the EPA regulations provided that means for removing enormous heat of adsorption generated when adsorbing gaseous hydrocarbons is perfect.
The heat of adsorption generated when gaseous hydrocarbons are adsorbed by use of activated carbon amounts to 10 to 15 kcal/mol, however. Waste gases released when loading a tank truck with gasoline amount to a few hundred cubic meters per hour, for example. An enormous amount of heat of adsorption would be generated if such waste gases whose gaseous hydrocarbon content is as high as 20 to 50 vol % are adsorbed by activated carbon.
If the means for removing this heat of adsorption is inadequate, the temperature of an activated carbon adsorption layer would rapidly increase, resulting in local heating and eventual firing or explosion. In fact, a number of such incidents have been reported in the past. The use of activated carbon makes such incidents even more dangerous since the activated carbon is a kind of adsorbent which is easily ignited. Another potential cause of these incidents is heat of polymerization generated when adsorbed heavy hydrocarbons are polymerized by a catalytic effect of an activated carbon adsorbent.
In these circumstances, a number of attempts have thus far been made to find out industrially viable means for removing enormous heat of adsorption when using the activated carbon adsorption method.
An example of prior art technology is to wind a coil for circulating cooling water around an activated carbon adsorption layer or to embed a coil within the activated carbon adsorption layer. These approaches would necessitate an enormous amount of cooling water, however, when a large amount of activated carbon is required.
This is because it is essential to maintain the internal temperature of the activated carbon adsorption layer to about 60.degree. C. or less for the sake of safety, and it is possible use only the sensible heat of the cooling water. An extremely large amount of cooling water is required since the heat of adsorption generated in the activated carbon adsorption layer is by far greater than the amount of heat that can be carried away by the cooling water.
To provide means for solving this problem, Japanese Examined Patent Publication No. 59-50716 proposed a method of using "an organic liquid (e.g., liquid gasoline) which boils at a temperature lower than 100.degree. C. close to room temperature" instead of water.
According to this method, it is possible to utilize the latent heat of vaporization of the organic liquid (e.g., liquid gasoline) and, therefore, the enormous heat of adsorption can be removed without requiring an unduly large amount of the organic liquid. This is because 1 kg of liquid gasoline, which is taken as an example for the purpose of explanation, removes nearly 100 kcal of latent heat when boiled. (Contrary to this, the sensible heat taken away by 1 kg of water is only 1 kcal/.degree. C.).
With the development of this means, the problem of "how to remove the enormous heat of adsorption" has been solved. However, it has raised questions relating to the safety of the adsorption layer.
Particularly because activated carbon is a material which is easily ignited, new controversy has arisen as to whether it is right or wrong to pass such a hazardous combustible substance like gasoline through a coil embedded in the activated carbon adsorption layer.
To overcome the aforementioned problem of the activated carbon adsorption method, an alternative method has been proposed which employs, as means for treating a large amount of waste gases containing highly concentrated gaseous hydrocarbons, a combination of the adsorption method and the conventionally used absorption method or membrane gas separation method, rather than treating the waste gases by the adsorption method alone.
In this combination method, the concentration of gaseous hydrocarbons entering an activated carbon layer is limited to a low level by performing absorption operation or membrane gas separation operation prior to activated carbon adsorption operation, and as a consequence, it becomes possible to reduce the heat of adsorption generated in the activated carbon layer to a lowest possible level. If depressurized water whose boiling point is 80.degree. C. or less is circulated as cooling water for removing the generated heat of adsorption, it also becomes possible to prevent a temperature increase due to local heating of the activated carbon layer and even out the temperature within the activated carbon layer.
Compared to the earlier-described prior art method which utilizes liquid gasoline in place of cooling water, this combination method is more safety-oriented as it provides means for achieving a uniform temperature distribution within the activated carbon layer while keeping the temperature to a low level.
This approach of combining the adsorption method with the absorption method or membrane gas separation method has drawbacks, however, in that such combination is somewhat uneconomical and overall process becomes complex.
Accordingly, it is an object of the invention to provide a method of treating gaseous hydrocarbons contained in waste gases which makes it possible to prevent abnormal temperature increase within each adsorbent layer, enhance the safety of adsorption apparatus and decrease the density of residual gaseous hydrocarbons in gases released into the atmosphere to 1 vol % or less.
Considering the above situations, the inventors carried out an intensive study and have completed the present invention.